Egg whites are as close to a pure amino acid supplement as you can get in a naturally occurring protein food source. Amino acids are the building blocks of protein, and egg whites are almost 100% protein—with no fat or cholesterol. The egg white in an average large egg contains approximately 3.4 grams of protein, with essential amino acids comprising 40% of the total amino acids. The egg white in an average egg contains only 16 calories. Pretty impressive!
Why Are Amino Acids Important?
The body is largely composed of thousands of proteins. Proteins are made of amino acids hooked together in a specific order and amount. All of the proteins in the body are in a constant state of turnover—older proteins are broken down and replaced with newly synthesized ones. During this process, amino acids are released. Many of these unchained aminos can then be reutilized to help make new proteins. However, others are irreversibly oxidized and therefore unavailable for protein synthesis.
There are two general types of dietary amino acids: those that can be produced in the body (called nonessential amino acids) and those that must be obtained through the diet (the essential amino acids). There are dietary requirements for each of the 9 essential amino acids, but there are no dietary requirements for the 11 nonessential amino acids, since they can be made in the body. You get additional nonessential amino acids from the protein foods you eat, because all dietary proteins that contain essential amino acids also contain nonessential amino acids. Essential amino acids, however, aren’t so easy to come by, and they are the only macronutrient required in the diet for survival.
Essential Amino Acids Define Protein Quality
Protein quality can be determined quantitatively by a scoring system developed by the Food and Agriculture Organization of the World Health Organization. The scoring system is called the Digestible Indispensable Amino Acid Score (DIAAS). It is based on the proportion of essential amino acids to nonessential amino acids in a protein, the profile of the essential amino acids relative to the individual requirements for the essential amino acids, and the digestibility of the protein.
Amino Acids in Egg Whites and Protein Quality
Egg white protein (largely a protein called albumin) has one of the highest DIAASs known. This is due not only to the fact that over 40% of the total amino acids are essential aminos, but also to the specific profile of the essential amino acids in egg whites.
The figure below shows the profile of essential amino acids in egg whites and egg yolk compared to the profile of essential amino acid requirements. You can see how the essential amino acid profile of an egg white is nearly a perfect match to essential amino acid requirements.
Then there’s the low-calorie benefit of egg whites. On average, the egg white of one large egg contains 3.4 grams of protein and 1.4 grams of essential amino acids (11 calories of essential amino acids) with no fat or cholesterol.
Are Egg Yolks the Villain?
It may be surprising, but egg yolks are also a great source of essential amino acids. The average egg yolk contains 2.7 grams of protein (compared to the egg white value of 3.4 grams), and the profile of egg yolk protein is also outstanding. You can see from the figure above that the profile of essential amino acids in egg yolk parallels that of essential amino acid requirements just as well as egg white protein does. Of course, the essential amino acids in the yolk come at a higher calorie cost, as there are approximately 46 calories in the average egg yolk, and approximately 44 kcal/g of essential amino acids in the yolk. The extra calories are largely in the form of fat. The yolk also contains about 160 milligrams of cholesterol.
You Can Eat the Whole Egg, but Don’t Overdo It!
It’s clear that both the white and the yolk of eggs are great sources of high-quality protein. Eating the whole egg is more convenient and tastes better to most people than separating out the egg white.
The argument against eating whole eggs has always been the fat and cholesterol in the yolk. The yolk adds extra calories, but the main worry has been related to the development of cardiovascular disease from the cholesterol in the yolk.
Attitudes are changing about cholesterol in the diet, and these changing attitudes have affected dietary recommendations for whole egg consumption. The American Heart Association recommends that total cholesterol intake be less than 300 milligrams per day. This is about what you get in two eggs, so if you eat two eggs a day you wouldn’t want to eat any other sources of cholesterol.
The USDA dietary recommendations are more lenient towards whole egg consumption. While the 2010 recommendations limited cholesterol intake to 300 milligrams per day, the 2015 recommendations drop the limit on dietary cholesterol. The USDA Dietary Guidelines state that eggs are good sources of high-quality protein and don’t specifically limit the number of whole eggs in the diet. In the healthy eating patterns recommended by the USDA, eggs are part of a balance of protein food sources.